Importance of boreal rivers in providing iron to marine waters

Abstract

This study reports increasing iron concentrations in rivers draining into the Baltic Sea. Given the decisive role of iron to the structure and biogeochemical function of aquatic ecosystems, this trend is likely one with far reaching consequences to the receiving system. What those consequences may be depends on the fate of the iron in estuarine mixing. We here assess the stability of riverine iron by mixing water from seven boreal rivers with artificial sea salts. The results show a gradual loss of iron from suspension with increasing salinity. However, the capacity of the different river waters to maintain iron in suspension varied greatly, i.e. between 1 and 54% of iron was in suspension at a salinity of 30. The variability was best explained by iron:organic carbon ratios in the riverine waters--the lower the ratio the more iron remained in suspension. Water with an initially low iron:organic carbon ratio could keep even higher than ambient concentrations of Fe in suspension across the salinity gradient, as shown in experiments with iron amendments. Moreover, there was a positive relationship between the molecular size of the riverine organic matter and the amount of iron in suspension. In all, the results point towards a remarkably high transport capacity of iron from boreal rivers, suggesting that increasing concentrations of iron in river mouths may result in higher concentrations of potentially bioavailable iron in the marine system.

Figure 1. Yearly mean iron concentrations in the river mouths of three different rivers from 1976–2012.

Lines denote the linear regression equations, which were µmol Fe L⁻¹ = 0.212× year – 413.2 (r² = 0.56, p<0.001); µmol Fe L⁻¹ = 0.477× year – 924.0 (r² = 0.40, p<0.001); and 0.716× year – 1398.4 (r² = 0.51, p<0.001) for Emån, Lyckebyån and Helgeån respectively.

Figure 2. Concentration of iron in suspension at different salinity after addition of artificial sea salt.

Figure 3. Differences in iron and organic matter in suspension in river waters at 0 and 30 salinity.

A) ratio between iron and organic carbon, B) ratio of absorbance at 465 and 665 nm, C) specific UV absorbance at 254 nm and D) fluorescence index (ratio of emission at 470 and 520 nm and an excitation of 370 nm).

Figure 4. Relationship between the fraction of initial iron remaining in solution (iron transport capacity) at a salinity of 30 and the molar iron:organic carbon ratio of the river water (r² = 0.54, p<0.05).

Figure 5. Concentration of iron in suspension at different salinity with and without iron amendments.